In recent years, extensive research and development works have been undertaken to provide a polymer electrode membrane type fuel cell. The polymer electrode membrane type fuel cell (PEFC) is capable of generating power under the room temperature conditions and has been coming into wide use. And this type of a fuel cell system does not require the compression of intake gas (air-fuel mixture), allowing the height of the whole system to be smaller than an internal combustion engine. Therefore, it can be disposed in a small space such as the under-floor of a vehicle compartment etc., and the space efficiency is improved.
This type of a fuel cell system comprises a cathode electrode and an anode electrode, which interpose a solid polymer membrane (electrode membrane). It drives an external load with the electrical power generated by the chemical reaction between the oxygen supplied to the cathode electrode and the hydrogen supplied to the anode electrode. However, because the chemical reaction for generating power is an exothermic reaction, a cooling system is essential to constantly perform a stable operation regardless of the output fuel cell.
As this type of a cooling system, a two-step heat radiation system is known, in which the heat generated during the power generation by the fuel cell is released by a radiator and the cooling line of the fuel cell is cooled through an intermediate heat exchanger. FIG. 10 shows said cooling system. As shown in FIG. 10, a cooling system 211 comprises; a cooling path 213 laid out in a fuel cell 212 to cool said fuel cell 212; a primary coolant circulating path 214 to circulate the primary coolant to the fuel cell 212; a secondary coolant circulating path 215 to cool the primary coolant circulating in said primary coolant circulating path 214; a heat exchanger 216 to cool the primary coolant by exchanging heat between the primary and secondary coolant. The inlet and outlet of said primary coolant circulating path 214 are respectively connected to the inlet and outlet of the cooling path 213 laid out in said fuel cell, and a radiator 217 is disposed within the secondary coolant circulating path 215. The secondary coolant cooled by said radiator 217 cools the primary coolant, thus cooling the fuel cell 212. A bypass path 218 bypassing the heat exchanger 216 is placed in the primary coolant-circulating path 214. A thermostat valve 220 is disposed at a connection 219 between said bypass path 218 and the downstream location of the primary coolant-circulating path 214 with regard to said heat exchanger 216. The switching of the thermostat valve 220 controls the primary coolant temperature to be appropriate for the power generation of the fuel cell 212. The primary and secondary coolants are a mixture of ethylene glycol and water, the ratio of the two fluids being determined as required.
It is necessary to humidify the electrode membrane to operate the fuel cell and there is a known device to collect the water for humidification (off-gas collection) from the generated water by the power generation of fuel cell. FIG. 11 depicts this type of a system. A cooling system 200 basically comprises; a coolant path 201c laid out in a fuel cell 201 to cool the same, a circulating path 202 and a circulating pump 202a connected to said coolant path 201c to circulate the coolant in the fuel cell 201, a heat exchanger 202b to cool the coolant, a thermo regulator 202c to control the temperature of the coolant supplied to the fuel cell 201. An outlet 202g and an inlet 202f of said circulating path 202 are connected to an inlet 201a and an outlet 201b of the coolant path 201c laid out in said fuel cell 201 and the fuel cell 201 is cooled by circulating the coolant with the circulating pump 202a. 
Generally speaking in the cooling system of fuel cell, pure water or dielectric coolant is employed as coolant for the fuel cell cooling to prevent the phenomenon of fluid short circuit. (An off-gas is discharged from the fuel cell 201 as a mixture of vapor and water, which may cause a short circuit with the structure supporting the fuel cell 201 through said water. This short circuit is referred to as “fluid short circuit”.)
In the circulating path 202, which is for the coolant of the cooling system of fuel cell 200, a bypassing path 202e is provided bypassing the heat exchanger 202b to directly supply the coolant to a thermoregulator 202c which is placed at a downstream in the bypassing path 202e when the coolant does not require cooling by the heat exchanger 202b. Thus, the cooling system 200 controls the coolant temperature appropriate for the power generation by the fuel cell 201.
Further, an ion exchanger 202d is provided in a bypassing path 202h, which connects the outlet-side path of the thermo regulator 202c and the upstream path of the circulating pump 202a, maintaining the low electrical conductivity of coolant.
In the cooling system 211 of fuel cell 212, circulating pumps 221 and 222 are provided for the primary coolant-circulating path 214 and the secondary coolant-circulating path 215 respectively to circulate the coolant. The circulating pumps 221 and 222 forcefully circulate the primary coolant and the secondary coolant respectively. However, when the vertical difference of the primary coolant-circulating path 214 is small and the gas venting of the path is not sufficient, the biting noise of the circulating pump 221 may occur.
It is feared that the intake pressure of the cooling path 213 varies and imposes the overload, which releases the contact of the separators, on the contact surface of cells of the fuel cell 212, namely the contact surface of the separators providing the cooling path 213, thus giving rise to the fluid leak and electrical conductivity failure of the coolant path 213, etc.
In said cooling system 200, when for instance the power output of the fuel cell 201 becomes large, the fluid pressure of the circulating path 202 will increase due to; (i) the coolant expansion by the temperature increase, (ii) the pressure loss inherent to the flow and (iii) the increase of operating pressure of the circulating pump 202a circulating the coolant. If the pressure of the coolant is too high, it may cause the following problems:                (1) As shown in FIG. 12, in a cell 300 of the fuel cell 201, the pressure of the coolant releases the contact surface S of separators 302, 302 providing the coolant path 201c, resulting in the cause of fluid leak or the electrical conductivity failure.        (2) When a humidifier 400 (humidifying by water chamber) with hollow fiber membranes 401, 401 shown in FIG. 13 is employed to humidify the gas supplied to the fuel cell 201, an excessive pressure will be imposed on the hollow fiber membrane 401 as the humidification is performed in a closed system.        
As countermeasures, following are anticipated to solve the excessive-pressure problem; (a) part of the cooling system releasing to the atmosphere, (b) regulating the pressure of the cooling system to a predetermined pressure by connecting to the atmosphere to breath through a pressure regulating valve and (c) regulating the pressure of the coolant by balancing the cathode gas pressure and the coolant pressure with the connection of the cathode gas tube and the cooling system.
However, because the pressure regulation is performed by introducing the air and the cathode gas, which are directly exposed to the coolant, the carbon dioxide in the air or the cathode gas will dissolve into the coolant, ionizing the coolant to increase the electrical conductivity (deteriorating the electrical insulation). Though the carbonate ion is absorbed by the ion exchange resin and the increase in the electrical conductivity of the coolant (insulation deterioration) will be prevented when an ion exchanger 202d shown in FIG. 11 is prepared in the cooling system, there still remains a problem that the life of the ion exchanger 202d is reduced as the operating hour of the ion exchange resin is lessened.
Humidifiers 203a, 203b are used to humidify the gas supplied to the fuel cell 201, which humidify through a membrane. Because there is no pressure regulating means in the circulating path 202 of the coolant used for humidification, the pressure of the coolant will not be regulated even if the output of the fuel cell 201 increases and the fluid pressure of the circulating path 202 gets higher.